This invention relates to a Magnetic Resonance Imaging (MRI) apparatus. More particularly, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field that makes nuclei, such as hydrogen atomic nuclei, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatial-gradient magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during the imaging pulse sequence. The radiofrequency coil(s) creates an excitation frequency pulse that temporarily creates an oscillating transverse magnetization that is detected by the radiofrequency coil and used by the computer to create the image.
Generally, very high field strength is characterized as greater than 2 Tesla (2 T). In recent years, there has been an increase in usage of MRI systems at field strengths above the typical 1.5 Tesla. Research systems have been built as high as 8 Tesla. Systems are now commercially available at 3 Tesla and 4 Tesla. The systems are primarily used for research in function MRI (fMRI) and human head related imaging and spectroscopy studies. Higher magnetic field strength imposes challenges on the RF coil, such as balancing inductance and capacitance at higher frequencies (greater than the typical 64 MHz). At very high magnetic fields, and therefore very high Larmor frequencies, standard birdcage coils with moderately narrow rung copper strips will have relatively high inductance requiring very low capacitor values in order to resonate the coil. This is problematic for a number of reasons. Firstly, high currents through small value capacitors will have very high voltage potential across them which can then have a local stray electric field that can dissipate RF power in the form of heat in an imaging subject. Alternatively, the stray electric field can dissipate RF power into lossy dielectrics such as a fiberglass bore tube structure of the MRI system, on which the RF coil is generally mounted. Secondly, a small value capacitor will have a higher interaction with and sensitivity to surrounding stray capacitance and can cause variability in the center frequency of the coil, as well as variability in the stability to resonate at the higher frequencies.
What is needed is a whole body RF coil for imaging other parts of the human body other than the head, as well as supporting the use of local receive only coils, that is effective at the high magnetic field strengths.
In a first aspect, a radio frequency (RF) coil assembly for a very high field Magnetic Resonance Imaging (MRI) system is provided comprising a plurality of conductors arranged cylindrically about a cylindrical patient bore tube of the MRI system and a plurality of capacitive elements for electrically interconnecting the plurality of conductors at respective ends of the conductors. The conductors have a width selected for the RF coil assembly to resonate at substantially high frequencies.
In a second aspect, a very high field Magnetic Resonance Imaging (MRI) system is provided that comprises a RF coil assembly adapted to resonate at substantially high frequencies, a RF coil shield assembly and a plurality of RF drive power cables.